Should update POS on unmapped reads in 'samtools depad'

Previously unmapped reads with a POS entry (e.g. those reads with
a mate which was mapped) were not being updated.

This has passed my testing with some non-trivial de novo MIRA 3.9
assemblies, coupled with the new 'samtools index' BIN verification.
Slightly more detailed error message for debugging.

It would be nice to include the RNAME, but currently the header
is being discarded before parsing.